TESTED ON AMPLITREX AT1000
ALL TUBES ARE TESTED ON MY AMPLITREX AT1000, WHICH IS A FULLY SERVICED, CALIBRATED MODERN DIGITAL TUBE TESTER.
FIXED BIAS MODE
*All my tubes are tested under Fixed bias mode *
B+ is applied to the tube and then a fixed bias voltage is applied to the grid. This fixed voltage is obtained from the stored tube test settings. the bias voltage is set to the specification value. Plate current will be whatever the tube is capable of, at this fixed grid voltage.
TEST RESULTS
LOOK AT PICTURES.
GOOD OR BAD TUBE?
You may ask, what constitutes a “good” tube? Well, this is difficult to
say, and depends more upon your own criteria. In the days when vacuum
tubes were the mainstay, most component values and supply voltages could
be off as much as 20%, and the equipment would still work
satisfactorily. You can apply this rule to a minimum acceptable Emission
and Transconductance of 20% low, or a display of 80%. Tubes showing
emission or GM less than about 70% can be classified as bad. However,
you must be the one to determine for yourself if a tube with 70%
emission or GM is suitable for use or not. For example, a tube showing
70% may perform very poorly in one piece of equipment, yet be OK in
Another.
GAS TEST – mA Gas
Most tubes have some noise so expect approximately up to 2.5 mA for even a unused tube. In radio receivers noise from a tube is likely to be less noticeable due to other factors. Compared to the desire for quality sound from audio amplifiers etc.
Preamp Tubes
In the case of a small preamp tube like a 12AX7 or EF86, anything
more than 0.1 milliampere of gas current will likely cause problems
in the application circuit. Noise may be excessive, and/or
the tube may not establish its correct operating point.
Power tubes
With power tubes a gasinduced
current increase of approximately less than 10 percent of normal plate current is OK in most application circuits.
Modern Manufacture Tubes
Some tubes of modern manufacture may show lower than “normal” grid
voltage (in Auto Bias Mode) or lower than normal plate current (in Fixed
Bias Mode), as some new tubes are not being manufactured to the same
standards as they were in the vintage years.
(Above text is for reference only. There are many opinions on preferred tube testing practices and how to interpret results. )
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